/****************************************************************************
program: proxyd
module: proxyd.c
summary: provides proxy tcp service for a host on an isolated network.

programmer: Carl Harris (ceharris@vt.edu)
date: 22 Feb 94

description:
This code implements a daemon process which listens for tcp connec-
tions on a specified port number. When a connection is established,
a child is forked to handle the new client. The child then estab-
lishes a tcp connection to a port on the isolated host. The child
then falls into a loop in which it writes data to the isolated host
for the client and vice-versa. Once a child has been forked, the
parent resumes listening for additional connections.

The name of the isolated host and the port to serve as proxy for,
as well as the port number the server listen on are specified as
command line arguments.
****************************************************************************/

#include <stdlib.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>
#include <netdb.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <sys/types.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <signal.h>
#include <sys/ioctl.h>
#include <sys/resource.h>
#include <sys/wait.h>

#define TCP_PROTO "tcp"

int proxy_port; /* port to listen for proxy connections on */
struct sockaddr_in hostaddr; /* host addr assembled from gethostbyname() */

extern int errno; /* defined by libc.a */
/*extern char *sys_errlist[];  defined by libc.a */


void parse_args (int argc, char **argv);
void daemonize (int servfd);
void do_proxy (int usersockfd);
void reap_status (void);
void errorout (char *msg);

 

/****************************************************************************
function: main
description: Main level driver. After daemonizing the process, a socket
is opened to listen for connections on the proxy port,
connections are accepted and children are spawned to handle
each new connection.
arguments:
argc,argv you know what those are.

return value: none.
calls: parse_args, do_proxy.
globals: reads proxy_port.
****************************************************************************/

main (argc,argv)
int argc;
char **argv;
{
 int clilen;
 int childpid;
 int sockfd, newsockfd;
 struct sockaddr_in servaddr, cliaddr;

 parse_args(argc,argv);

 /* prepare an address struct to listen for connections */
 bzero((char *) &servaddr, sizeof(servaddr));
 servaddr.sin_family = AF_INET;
 servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
 servaddr.sin_port = proxy_port;

 /* get a socket... */
 if ((sockfd = socket(AF_INET,SOCK_STREAM,0)) < 0) {
  fputs("failed to create server socket",stderr);
  exit(1);
 }

 /* ...and bind our address and port to it */
 if (bind(sockfd,(struct sockaddr *) (&servaddr),sizeof(servaddr)) < 0) { 
  fputs("faild to bind server socket to specified port",stderr);
  exit(1);
 }

 /* get ready to accept with at most 5 clients waiting to connect */
 listen(sockfd,5);

 /* turn ourselves into a daemon */
 daemonize(sockfd);

 /* fall into a loop to accept new connections and spawn children */
 while (1) {
 
  /* accept the next connection */
  clilen = sizeof(struct sockaddr_in);
  newsockfd = accept(sockfd, (struct sockaddr *) (&cliaddr), &clilen);  
  if (newsockfd < 0 && errno == EINTR)
  continue; /* a signal might interrupt our accept() call */
  else if (newsockfd < 0)
  /* something quite amiss -- kill the server */
  errorout("failed to accept connection");
  
  /* fork a child to handle this connection */
  if ((childpid = fork()) == 0) {
   close(sockfd);
   do_proxy(newsockfd);
   exit(0);
  }
  
  /* if fork() failed, the connection is silently dropped -- oops! */
  
  close(newsockfd);
 }
}

/****************************************************************************
function: parse_args
description: parse the command line args.
arguments:
argc,argv you know what these are.

return value: none.
calls: none.
globals: writes proxy_port, writes hostaddr.
****************************************************************************/

void parse_args (argc,argv)
int argc;
char **argv;
{
 int i;
 struct hostent *hostp;
 struct servent *servp;
 unsigned long inaddr;
 struct {
  char proxy_port [16];
  char isolated_host [64];
  char service_name [32];
 } pargs;


 if (argc < 4) {
  printf("usage: %s ",argv[0]);
  exit(1);
 }

 strcpy(pargs.proxy_port,argv[1]);
 strcpy(pargs.isolated_host,argv[2]);
 strcpy(pargs.service_name,argv[3]);

 for (i = 0; i < strlen(pargs.proxy_port); i++)
 if (!isdigit(*(pargs.proxy_port + i))) break;

 if (i == strlen(pargs.proxy_port)) 
  proxy_port = htons(atoi(pargs.proxy_port));
 else {
  printf("%s: invalid proxy port",pargs.proxy_port);
  exit(0);
 }

 bzero(&hostaddr,sizeof(hostaddr));
 hostaddr.sin_family = AF_INET;
 if ((inaddr = inet_addr(pargs.isolated_host)) != INADDR_NONE)
  bcopy(&inaddr,&hostaddr.sin_addr,sizeof(inaddr));
 else if ((hostp = gethostbyname(pargs.isolated_host)) != NULL)
  bcopy(hostp->h_addr,&hostaddr.sin_addr,hostp->h_length);
 else {
  printf("%s: unknown host",pargs.isolated_host);
  exit(1);
 }

 if ((servp = getservbyname(pargs.service_name,TCP_PROTO)) != NULL)
  hostaddr.sin_port = servp->s_port;
 else if (atoi(pargs.service_name) > 0)
  hostaddr.sin_port = htons(atoi(pargs.service_name));
 else {
  printf("%s: invalid/unknown service name or port number",pargs.service_name);
  exit(1);
 }
}

/****************************************************************************
function: daemonize
description: detach the server process from the current context,
creating a pristine, predictable environment in which it
will execute.
arguments:
servfd file descriptor in use by server.

return value: none.
calls: none.
globals: none.
****************************************************************************/

void daemonize (servfd)
int servfd;
{
 int childpid, fd, fdtablesize;

 /* ignore terminal I/O, stop signals */
 signal(SIGTTOU,SIG_IGN);
 signal(SIGTTIN,SIG_IGN);
 signal(SIGTSTP,SIG_IGN);

 /* fork to put us in the background (whether or not the user
 specified '&' on the command line */

 if ((childpid = fork()) < 0) {
  fputs("failed to fork first child",stderr);
  exit(1);
 }
 else if (childpid > 0)
  exit(0); /* terminate parent, continue in child */

 /* dissociate from process group */
 if (setpgrp(0,getpid()) < 0) {
  fputs("failed to become process group leader",stderr);
  exit(1);
 }

 /* lose controlling terminal */
 if ((fd = open("/dev/tty",O_RDWR)) >= 0) {
  ioctl(fd,TIOCNOTTY,NULL);
  close(fd);
 }

 /* close any open file descriptors */
 for (fd = 0, fdtablesize = getdtablesize(); fd < fdtablesize; fd++)
 if (fd != servfd) close(fd);

 /* set working directory to / to allow filesystems to be unmounted */
 chdir("/");

 /* clear the inherited umask */
 umask(0);

 /* setup zombie prevention */
 signal(SIGCLD,reap_status);

}

/****************************************************************************
function: do_proxy
description: does the actual work of virtually connecting a client to
the telnet service on the isolated host.
arguments:
usersockfd socket to which the client is connected.

return value: none.
calls: none.
globals: reads hostaddr.
****************************************************************************/

void do_proxy (usersockfd)
int usersockfd;
{
 int isosockfd;
 fd_set rdfdset;
 int connstat;
 int iolen;
 char buf [2048];

 /* open a socket to connect to the isolated host */
 if ((isosockfd = socket(AF_INET,SOCK_STREAM,0)) < 0)
  errorout("failed to create socket to host");

 /* attempt a connection */
 connstat = connect(isosockfd,(struct sockaddr *) &hostaddr,sizeof(hostaddr));
 switch (connstat) {
  case 0:
   break;
  case ETIMEDOUT:
  case ECONNREFUSED:
  case ENETUNREACH:
   strcpy(buf,sys_errlist[errno]);
  strcat(buf,"");
  write(usersockfd,buf,strlen(buf));
  close(usersockfd);
  exit(1); /* die peacefully if we can't establish a connection */
  break;
 default:
  errorout("failed to connect to host");
 }


 /* now we're connected, serve fall into the data echo loop */
 while (1) {
 /* Select for readability on either of our two sockets */
  FD_ZERO(&rdfdset);
  FD_SET(usersockfd,&rdfdset);
  FD_SET(isosockfd,&rdfdset);
  if (select(FD_SETSIZE,&rdfdset,NULL,NULL,NULL) < 0)
   errorout("select failed");

  /* is the client sending data? */
  if (FD_ISSET(usersockfd,&rdfdset)) {
   if ((iolen = read(usersockfd,buf,sizeof(buf))) <= 0)
   break; /* zero length means the client disconnected */

   write(isosockfd,buf,iolen); /* copy to host -- blocking semantics */
  }

  /* is the host sending data? */
  if (FD_ISSET(isosockfd,&rdfdset)) {
   if ((iolen = read(isosockfd,buf,sizeof(buf))) <= 0)
   break; /* zero length means the host disconnected */

   write(usersockfd,buf,iolen); /* copy to client -- blocking semantics */
  }
 }

 /* we're done with the sockets */
 close(isosockfd);
 close(usersockfd);
}

/****************************************************************************
function: errorout
description: displays an error message on the console and kills the
current process.
arguments:
msg message to be displayed.

return value: none -- does not return.
calls: none.
globals: none.
****************************************************************************/

void errorout (msg)
char *msg;
{
 FILE *console;

 console = fopen("/dev/console","a");
 fprintf(console,"proxyd: %s",msg);
 fclose(console);
 exit(1);
}

/****************************************************************************
function: reap_status
description: handle a SIGCLD signal by reaping the exit status of the
perished child, and discarding it.
arguments: none.
return value: none.
calls: none.
globals: none.
****************************************************************************/

void reap_status ()
{
 int pid;
 union wait status;

 while ((pid = wait3(&status,WNOHANG,NULL)) > 0); /* loop while there are more dead children */
}


